Hoppers are used for storing free-flowing, dry, particulate materials, such as grains and hydrated lime, the hopper having an outlet in the bottom and a funnel for channeling the dry particulate material to the outlet, through which the dry particulate material can be removed or withdrawn. The particulate material, stored or loaded in the hopper, can be periodically, intermittently, or continually removed through the hopper outlet when desired. Sometimes a dry particulate material, for various reasons, agglomerates or bridges in the hopper and can not be readily removed or may not be able to be removed at all from the hopper, that is, the material, in whole or in part, does not freely flow, and the bulk density of the removed material thus is not uniform or constant. In a hopper fitted with a volumetric feeder (a device that removes or withdraws material from a storage container such as a hopper at a constant rate of volume per unit time), variabilities of bulk density of the material removed from the hopper is particularly undesireable and troublesome.
Many dry particulate materials are shipped in large bags (sometimes containing as much as five thousand pounds of material). During shipment, because of vibration, time delay, and the weight of the material in the bags, the dry particulate material tends to settle in the bags, therefore changing the bulk density. Furthermore, because of the difference in pressure by the weight of material in different locations in a bag, the bulk density varies even for the same material in different locations in the same bag. Occasionally, moisture may get into the bag, resulting in the formation of lumps or aggregates. Such lumps may further affect the consistency of the bulk density as well as hinder the flow of the material in a hopper, or a volumetric feeder connected herewith, when the hopper is being emptied.
Before the dry particulate material is sold to the consumer, such material is often transferred from the large shipment bags into smaller containers. Typically a hopper having a volumetric feeder, such as an auger connected to the outlet of the hopper, is used for transferring the dry particulate material. Because a volumetric feeder feeds a fixed volume of material per unit period of time, variations in bulk density are undesirable, as mentioned above. Further, some dry particulate materials have a tendency to bridge in the hopper of the volumetric feeder, resulting in inconsistent feed rate and sometimes cessation of transferring of the dry particulate material out of the feeder. Conventional means of preventing bridging and facilitating the transfer of material out of a feeder hopper, such as using a vibrator on the walls of the feeder hopper, sometimes are unable to prevent bridging or to maintain a consistent bulk density of the material being transferred out of the feeder hopper.